There is no true standard for this. It's up to each manufacturer as it seems most of them have a setting to restore last known settings after power off upon power up. (ie: restore throttle and direction on startup)
Otherwise it's just a NC packet.
Don - Specializing in layout DC->DCC conversions
Modeling C&O transition era and steel industries There's Nothing Like Big Steam!
i think there are two cases:
A DC locomotive is only going to respond to DC. On a DCC layout the average voltage is normally 0. Some systems use address 0 to control a DC locomotive and will stretch pulses to provide a DC voltage to drive a single DC locomotive.
A DCC locomotive on a DC layout has limitations. As others have said, a sufficient voltage is required for the processor to run. If it doesn't see a DCC signal (presumably preamble) it operates in DC mode.
Since the motor is connected to an H-bridge and the track voltage is rectified, the decoder must determine the polarity of the track voltage and switch the H-bridge to provide the proper polarity to the motor
When a DCC decoder sees preamble, it waits for a packet with its address and continues to execute the last command which it may not have received yet and remains at zero throttle step.
greg - Philadelphia & Reading / Reading
I happen to have a Mantua 0-6-0 tank switcher sitting on my desk that I got on ebay (sold as not working - turns out it has a DZ125 decoder in it). It had analog enabled though, it ran fine with a plain power pack. I just tested it with my lab power supply, it starts turning the wheels at about 4.5 volts. I don't think these things were much better with no decoder. It's also tough trying to hold the loco upside down, hold a pair of banana plugs against the wheels, and adjust the power supply at the same time, but upside down freely spinning it was only drawing about 150ma.
Decoders will vary, depending on the microcontroller used, even with a nominal voltage spec of 5V they often run down to 4 volts or so. I'll have to dig out some of my other decodered locos and see how they start. And grab my Zephyr, my oscilloscope does screen caps so I can get a picture of the DCC waveform.
Sound decoders, if you want the sounds to start before the loco moves, need to get above 5V before the motor is powered so the sound processor can start and play the stationary sounds and/or run the startup sounds.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
When running on DC, the decoder microprocessor needs at least five volts to wake up and maybe a couple volts more for the motor begins to respond. Nature of the beast.
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
Ones and zeros.
http://www.nmra.org/sites/default/files/s-92-2004-07.pdf
Far more info than I will ever need.
When you place a dual-mode equipped loco on track, it will either be presented with high-frequency AC (DCC) or DC (DC). It's not hard for the decoder to see what it's getting and act accordingly.
If you put it on the AC, it will wait for an information packet superimposed on the AC that is addressed to it. If it gets one, it will act accordingly. If there is not such a packet, it will do nothing. It is a simple minded little fellow, and is not hurt or resentful if it sits on that track for a week with absolutely nothing to do. However, it might hum to itself a bit to pass the time.
If you put it on the DC, the decoder will switch the incoming voltages over directly to the motor leads. Sorta. The decoder will probably suck up a little power while doing this trick, but not much. And likely get bored with nothing to "decode". But being simple minded, boredom is not really such a bummer. I'm not sure if it hums to itself in DC. Probably not. Maybe it counts electrons.
In my group of ne'er-do-wells and other fine folks who operate with DCC only, people who know have told me that I will regret not switching off the automatic DC capability. Since those ne'er-do-wells, etc., are also very smart and experienced in DCC, I follow their direction.
Ed
In the link that 7j43k posted, there is a picture of what a Scope would show the instant a DCC system is powered up.
I know the Bachmann EZ Command does that.
A DCC system that uses stretch zero for DC locos would look a little different. You would see a positive or negative pulse in the waveform if the DC supply is powered up.
That is why a DC only loco will buzz as soon as the DCC system is powered up. The AC component is trying to power the DC motor which will only vibrate at the DCC frequency.
Use to happen to me when I used stretched zero bit option.
I have taken a few pictures with my camera that show essentially the same.
I don't have the test equipment to break it down anymore but there will always be a “AC” type waveform present even with no commands. The data is buried in the DCC signal.
Good enough for me.
I probably not sure what you are looking for.
http://members.shaw.ca/sask.rail/dcc/DCC-waveforms/DCC_waveforms.html
you've got me curious. Maybe the answer is in the NMRA DCC standards
Thanks, I appreciate the help. I've read that document and (others) but I haven't found an answer to my question yet. The documents describe how to create a DCC signal (specs of the 1's and 0's), and there is the standards documentation on the syntax for the packets. What I'm wondering is when you first apply power to the track and no commands have been pressed, what is going through the rail at that moment? Whatever it is, the decoder looks at it and decides whether to go into DCC mode or DC mode. Is there some sort of placebo packet that the command stations deliver initially? Is a series of 0's sufficient to keep a decoder from switching to analog mode (before any commands are sent)? I'll order a packet analyzer and see what I can see, but it might take a while to arrive. Thanks again.
Yea, just found that one. If the OP wants a little more, just Google DCC waveforms. Works better than DCC signals.
Be advised, at least five other systems use digital control through the track or wireless that are not DCC. DCS is similar.
DCC is the NMRA version.
Wikipedia should have all of them.
This will probably answer your questions. And others you haven't thought of yet.
http://www.dccwiki.com/DCC_Power
When you first power up your DCC system and have not yet pressed any commands, what does the DCC signal in the track look like? Are there any packets being sent? Or is it just a series of 1's? or a series of 0's?
I know many decoders have " dual mode" which allows them to run on DCC or DC. I'm trying to understand what they are looking for in order to decide whether to go into DCC mode or DC mode. Will a series of 0's serve well enough to constitute a DCC signal (in the eyes of the decoder)?